454 diesel effect

[Poprivet]

Ok you big block nuts, I have a 1990 classic with a carburated 454 and she runs strong but lately she runs a high idle, diesels for a min or two, and loudly backfires like a gun shot. Last time I opened the gas filler cap, I noticed pressure released?? Any ideas?

[RichardT]

Check your timing, see if the distributor clamp has become loose. Just try to turn it and if it moves you need to tighten the clamp after timing it. Also check for vaccuum leaks which would account for the high idle too.

Good luck
RichardT

[ticki2]

couple of things to check that will cause " dieseling".

Idle set too high - check linkage and carb adjustment

Timing too far advanced

Engine too hot

Octane rating too low , try high test

Carbon deposits on valves or seats

Dirt in carb

Coke stuck partially on

These are just some

[vswingfield]

My ’73 GMC with a 454 had a solenoid on the throttle that held it open to idle speed. When you shut off the ignition, it would drop closing the blades. It was specifically to prevent dieseling after shut down. Does your have anything like that? Just a thought.

[jdalrymple]

I agree with timing advanced or more likely, a cracked vaccuum line or fitting somewhere. A little WD40 sprayed along the lines will cause the idle to increase at the leaking point.

Good Luck!